FINDING A TITLE to anchor a thematic group show is notoriously fraught, as “The Shadow of the Avant-Garde: Rousseau and the Forgotten Masters” demonstrated. German art historian Veit Loers coined the titular phrase to refer to a corpus that is the necessary complement to the art of the historical avant-gardes, the negative or occluded partner in the pairingnamely, the work of those non-Europeans, “folk,” children, and other “primitives” whom the early modernists “discovered.” The show’s subtitle indicated that the exhibition would focus on the renowned Henri Rousseau, the first and still